1. Field of the Invention
The present invention relates to a water-hardenable molding material that has a high heat resistance, good heat insulation, high resistance to molten metals, good electrical insulation and good machinability and which provides moldings that are advantageously used as heat insulators or electrical insulators. The molding material of the present invention is particularly useful in the manufacture of lining materials and melt receptacles for use with low-melting point metal casting apparatus.
2. Background of the Invention
Castable refractories are extensively used as lining materials in manufacturing, constructing or repairing runners, melt holding furnaces, ladles and other members that come in contact with the melt in an apparatus for casting aluminum or other metals having comparatively low melting points. After being mixed with an appropriate amount of water, castable refractories are poured into a form work, hardened, dried and fired to produce linings that have no adhesive water or water of crystallization. The so prepared linings are highly refractory and will not generate any steam during service.
Castable refractories that have heretofore been used as the material of linings on low-melting point metal casting apparatus are mostly based on alumina cements that are not easily wettable with molten aluminum or other metals and which have fairly good resistance to corrosion. However, the lining materials made of conventional aluminous-cement based castable refractories are not completely satisfactory in terms of flexural strength. In addition, if the hydrate of aluminous cement (i.e., calcium aluminate cement) undergoes rapid reactions of decomposition and dehydration either during firing that is effected before reception of molten metal or as a result of temperature increases following contact with a molten metal that occurs during service, the lining will readily experience a reduction in strength and cracking due to excessive shrinkage may easily occur. In order to avoid these problems, the firing of the lining materials prior to reception of molten metal has to be accomplished by slowly elevating the temperature over a period that usually lasts for one to two weeks. A further problem of the prior art lining material is that it is difficult to form products of complex shapes.